New study pinpoints how lunar missions can trigger moondust

By the end of this decade, NASA plans to return astronauts to the moon for the first time since the Apollo era. But this time, by the way Project Artemis, it will not be a „tracks and flags” affair. Along with other space agencies and commercial partners, the long-term goal is to build infrastructure that allows for a „sustained program of lunar exploration and development.” If all goes as planned, several space agencies will establish bases around the South Pole-Aitken Basin, paving the way for lunar industries and tourism.

For humans to live, work, and carry out various activities on the Moon requires strategies to deal with all hazards – not least of which is lunar regolith (or “moondust”). As the Apollo astronauts learned, lunar dust is dislodged and sticks to everything, and can cause significant wear and tear on astronauts' clothing, equipment, vehicles, and health. A A new study According to a team of Texas A&M engineers, regolith also poses a collision risk when kicked up by rocket plumes. With many spacecraft and landers expected to deliver crews and cargo to the Moon in the near future, this is a risk to be very careful of!

The study was conducted by Shah Akib Sarwar And Johaib Hasnain, a Ph.D. J. from Texas A&M University. student and assistant professor (respectively) of Mike Walker '66 in the Department of Mechanical Engineering. For their study, Sarwar and Hasnain investigated the Moon's particle-particle collisions using the „soft sphere” method, where Newton's equations of motion and a contact force model are combined to study how particles collide and collide with each other. This distinguishes it from the „hard sphere” method, which models particles in environments of fluids and solids.

An Apollo 15 astronaut salutes next to the American flag in 1971. The Moon's regolith, or soil, appears in various shades of gray. Credit: NASA

While lunar regolith ranges from small particles to large rocks, the main component of „Moondust” is fine, silicate minerals with an average size of 70 microns. These were formed over billions of years, when meteorites and asteroids struck the airless surface of the moon, pulverizing much of the lunar crust into fine powder. Because there is no atmosphere there is no erosion by wind and water (common here on Earth). Finally, the lunar regolith is electrically charged due to constant exposure to the solar wind, meaning it adheres to anything it touches.

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When the Apollo astronauts went to the moon, they reported problems with regolith clinging to their suits and being tracked back into their lunar modules. Once inside their vehicles, it sticks to everything and is harmful to health, causing eye irritation and respiratory problems. But with the Artemis missions on the horizon and the planned infrastructure, the problem remains how the spacecraft (during take-off- and landing) will kick up the regolith in bulk and accelerate to high speeds.

As Sarwar related to Universe Today via email, this is one of the main ways that lunar regolith poses a major challenge to routine human activities on the Moon:

„During a retro-propellant soft landing on the Moon, supersonic/hypersonic rocket exhaust plumes can eject a large volume (108 – 1015 particles/m3) loose regolith from the upper soil layer found on the Apollo missions. Plume-generating forces – drag, lift, etc. – can cause the plume to travel at very high speeds (up to 2 km/s). The spray can harm the spacecraft and nearby equipment. It can block the view of the landing area, disrupt sensors, clog mechanical components, and degrade optical surfaces or solar panels through contamination.

Data obtained from the Apollo missions served as a touchstone for Sarwar and Hasnain, in which exhaust plumes were ejected from the exhaust plume. Apollo 12 Chandra block (LM) damaged Surveyor 3 The spacecraft is located 160 meters (525 feet) away. In 1967 the unmanned rover was sent to Mare Cagnes to explore the area and characterize the lunar soil ahead of crewed missions. Surveyor 3 was also used as a landing target platform Apollo 12 Astronauts Pete Conrad and Alan Bean visited in November 1969.

Check out the Apollo 12 landing pad. Astronaut Alan Bean is shown working near the Modular Equipment Stowage Assembly (MESA) on the Apollo 12 Lunar Module (LM) during the mission's first Extravehicular Activity (EVA) on November 19, 1969. Credit: NASA.

The damage was mitigated by the fact that Surveyor 3 was sitting in a ditch Below the Apollo 12 LM landing pad. Another example Apollo 15 1971 Landing mission in the Hadley-Apennine region. During the descent of the LM, astronauts David R. Scott and James B. Irwin could not see the landing site because their exhaust plume had created a thick cloud of regolith above it. This forced the crew to select a new landing site on the edge of Pela, an elongated crater to the east of the region. The LM was unable to achieve a stable position at this site and tilted back 11 degrees before settling itself.

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Research since these works has led to the conclusion that the scattering may have been caused by collisions between regolith particles. As Sarwar noted, these examples illustrate how disturbed regolith can become a hazard, especially where other spacecraft and facilities are located nearby:

„The above two Apollo-era examples are not severe enough to affect mission success. But future Artemis (and CLPS) missions will take place at the South Pole of the Moon, where soil is considered to be significantly more porous/weak than the equatorial and mid-latitude Apollo landing regions. Also, the Artemis landers will be able to use the Apollo are expected to deliver much larger payloads than expected, so more thrust is required to decelerate. As a result, rocket exhaust plumes can cause deeper cratering (not seen on Apollo) and blow regolith at greater angles than previously seen (~1-3 degrees from the ground).”

In keeping with the long-term goals of the Artemis program, NASA plans to build infrastructure around the South Pole. This includes Artemis Base CampA Base Surface Habitat, a Habitable Mobility Platform, a Lunar Terrain Vehicle (LTV) and Lunar Gateway In orbit. „As such, it is critical to protect humans, structures or nearby spacecraft from the hazards of lunar regolith particles,” Sarwar said.

Base of the Moon
Description of NASA astronauts at the lunar south pole. The mission ideas we see today have at least some heritage from the early days of the space age. Credit: NASA

Shows how regolith clouds from landings and takeoffs can pose a threat to the safe operation of lunar entry and lunar orbiters. These threats have driven considerable research into how to reduce lunar dust during future missions. As mentioned, Sarwar and Hasnain used the smooth sphere method to estimate the risks from particle-particle collisions:

„In this method, adjacent particles are allowed to overlap by a small amount, which is taken as an indirect measure of the deformation expected in a real particle-particle collision. This overlap value, along with the corresponding material properties of the lunar regolith, is a spring-dashpot-friction slider to calculate the forces in each collision event. Used in representation.The elasticity involved in the collision varies from completely inelastic to highly elastic.

„Our results reveal that most elastic collisions occur between relatively large regolith grains (~100 microns). However, the rest of the grains are at a small angle to the ground (<3 degrees) - consistent with the regolith sheet seen during the Apollo missions.

As for safeguards, Sarwar and Hasnain suggested that berms or fences around the landing zone could be a way to mitigate ejecta sprays. However, as their research suggests, a certain percentage of regolith particles may scatter at large angles due to collisions, berns, or insufficient fences. „The landing pad will be developed as an ideal solution for future Artemis missions,” Sarwar said. „In this regard, a multi-institutional team consisting of academics (including Dr. Hasnain) and personnel from industry is working on developing an in-flight alumina spray technique or fast landing pads.”

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The FAST method envisions lunar landers with alumina particles ejected during the landing maneuver. They are then liquefied by engine plumes to form molten aluminum on the lunar surface, which cools and solidifies to form a stable landing surface. NASA has also studied how to create landing pads using sintering technology, where regolith is blasted by microwaves to form molten ceramics. Another idea is for a Texas-based construction company to build landing pads with blast walls that include extruded regolith. Icon Their lunar lamprey is included in habitat considerations.

Illustration of In-Air Alumina Spray Technique (FAST). Credit: Maston Space Systems

Alas, experimental studies of the lunar regolith are extremely difficult because the conditions on the moon are very different from those on Earth. This includes low gravity (approximately 16.5% of Earth's), a vacuum environment, and extreme temperature variations. Hence why researchers are forced to rely heavily on numerical modeling, which typically focuses on plume forces and largely ignores the role of particle collisions. But as Sanwar noted, their research provides valuable insight and explains why it's important to consider this often-overlooked phenomenon when planning future lunar missions:

„[However,] Our research on particle collisions shows that this is a very important phenomenon to consider for accurate regolith trajectory prediction and, therefore, should be included. There are still many challenges in this area, such as the regolith particle recombination coefficient (which determines the energy loss in collision), the effects of regolith size distribution, the effects of turbulent plumes, etc. Let's clarify a few things. These uncertainties in the future contribute to a more comprehensive lunar PSI model for a safe Artemis lunar landing.

read more: Acta Astronautica

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